Distant Atoms Cooperate by Sharing Light

CALGARY, Alberta, and SHERBROOKE, Quebec, Canada & ZURICH, Nov. 14, 2013 — Atoms can work collectively, rather than independently, to share light, an international team has found. The discovery has important implications for applications in advanced quantum devices.

Researchers demonstrated light sharing, or "photon-mediated interaction," between artificial atoms confined to a one-dimensional quantum system. Quantum physicists have long discussed such a possibility, but this is the first experimental observation of the effect. The team included scientists from ETH Zurich, who performed the experiment, and theoretical scientists from the Université de Sherbrooke in Quebec and the University of Calgary in Alberta.

The chip containing the superconducting circuits used in the experiment had a design identical to this one. Courtesy of Barry Sanders.
Getting artificial atoms to work collectively could lead to control of microwave fields in superconducting circuits with benefits, including ways to protect quantum information against "noise" or signal damage, said Barry Sanders, professor of physics and astronomy at the U of C and director of the university's Institute for Quantum Science and Technology.

"I think what we've shown is going to be critical for future applications. It's an unobserved effect that has been discussed for decades, and we see it with excellent agreement between theory and experiment," Sanders said.

The two artificial atoms “showed a coherent exchange interaction, something not seen before for distant quantum systems in an open environment,” said Arjan van Loo, a Ph.D. student in the Quantum Device Lab at ETH Zurich.

The key to the approach was to do the experiment in one dimension rather than in three, where the interaction between atoms is weak and declines significantly with distance.

Realizing such fundamental interactions between individual quantum systems in one dimension is crucial to advancing quantum-based devices, and is expected to be useful for routing quantum information along one-dimensional waveguides on devices used for quantum information processing or quantum communication, they said.

This illustration shows the artificial atoms or "qubits" alongside the one-dimensional waveguide, or transmission line. Courtesy of Barry Sanders.
This research shows that "man-made electrical circuits can now be engineered in such a way to exhibit behavior that is not possible in 'natural' quantum systems," said Alexandre Blais, associate professor of physics at Université de Sherbrooke.

Confining two artificial atoms to one dimension using a waveguide greatly increased the possibility that the two systems would interact and enabled measurement of the interaction. Using superconducting circuits, the team put two artificial atoms alongside the waveguide and sent a microwave field through the waveguide.

At a distance of approximately 2 cm – much larger than typically expected for quantum systems – the two atomlike systems formed a type of weakly bound molecule, due to the exchange of photons.

"We also observed how the superconducting circuits either synchronize to emit radiation much more efficiently displaying superradiance, or how the circuits trap radiation, turning the two systems dark, as they do not emit photons anymore," said Andreas Wallraff, professor of solid-state physics at ETH Zurich.